Current standards for wireless communications systems, e.g. Long Term Evolution (LTE), support flexible bandwidth: from 1.4 MHz up to 20 MHz and also wider bandwidths using carrier aggregation techniques. In order for a communication device, exemplified in the following by a user equipment (UE), to connect to a network node (e.g. eNodeB in case of LTE) the UE needs to determine the cell carrier frequency as well as the system bandwidth to use. Furthermore, in current LTE standards there is a requirement for the network node and the UE to support and connect using the same system bandwidth. Hence the UE must search for e.g. control messages over the entire system bandwidth of the network node.
For the upcoming new radio-access technology in 5G, denoted NR herein, a more generic approach is desirable with respect to the system bandwidth of the respective network nodes. NR should support several different types of UEs/devices, from high end mobile broadband UEs capable of up several GHz system bandwidth, down to low-cost low-power Machine-Type Communications (MTC) devices, possibly only supporting some too kHz up to some MHz bandwidth. Hence a desired requirement is that the UE, supporting for instance a 100 MHz bandwidth, could be allocated a dedicated UE system bandwidth, denoted scheduling bandwidth, anywhere within the total system bandwidth of a next generation base station (denoted gNodeB). As a particular example, the scheduling bandwidth may be maximum 100 MHz while the total system bandwidth may be 1000 MHz. It is also desirable that the gNodeB can allocate a smaller scheduling bandwidth than the one supported by the UE.
In NR it is proposed that the UE relies on synchronization signals for cell detection. The synchronization signals will not fill up the entire system bandwidth of the gNodeB, but instead only a subband (i.e. only part of the bandwidth). The subband will be configured by the serving network node/serving beam, that informs adjacent nodes which subband to use. The UE is also informed about the subband to use in order to know where to find the synchronization signals.
In NR there might not be fixed location (i.e. fixed location in time and/or frequency) of the synchronization signals as in the LTE legacy system. Hence the UE will have to search both in frequency and time within the system bandwidth when searching for intra-frequency neighbor cells.
In the LTE legacy system, synchronization signals are transmitted over six central resource blocks (RBs) (1.4 MHz) every 5 milliseconds (ms), and hence for cell detection as well as mobility measurements it is sufficient to operate on radio samples at a sampling rate of 1.92 MHz, although the system bandwidth might be 20 MHz by which radio samples are acquired at a 30.72 MHz sampling rate. This allows UE implementations to record and post-process radio samples for cell detection and mobility measurements, where the post-processing can be carried out when physical resources such as hardware (HW) accelerators and Digital Signal Processors (DSPs) are idling. This results in a lower UE complexity than if the UE would need to carry out all operations, communication tasks as well as cell detection, in real-time.
With the increased repetition period of synchronization signals in NR compared to LTE (for instance, too ms instead of 5 ms), and the flexibility with respect to which subband the synchronization signal is transmitted in, it becomes challenging for the UE to detect intra-frequency neighbor cells without a dramatic increase in UE complexity, in terms of memory requirements, processing capabilities, or both, compared to current LTE.
From the above it is realized that there is a need for NR cell detection without increasing complexity and hence cost of the communication devices.